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Re: Breathing

At 03:42 PM 4/17/99 -0400, Patrick Norton wrote:
>although I remain curious about the evolutionary and physiological factors
>that apparently *precluded* (?) in terrestrial vertebrates the true
>counterflow respiratory systems (aka gills) seen in more primitive

It is likely a combination of excessive drying in exposed gills and a
tendency of gills to collapse (loosing surface area) when not supported by

>>>The oldest known jawed vertebrates had *both* structures! <<
>The condition of being a "jawed" vertebrate is not one I made, but the point
>is taken.

(I added the qualifier mostly because I am unsure of he condition in

> I still hold to my conditional acceptance of the "previous
>anatomy" argument, however, since, if the vertebrate lung is not derived
>from the gill, at some point in vertebrate evolution an innovation had to
>occur to produce the terrestrial vertebrate lung.

Well, sort of.  Lungs originated in fish, for fishy purposes, and were
simply later co-opted for terrestrial gas exchange.  I do seem to remember
that gills are slightly older structures, but lungs still long predate the
move to land.  "Lungfish" are very old indeed.  Indeed, the swim bladder of
living teleost fish is *derived* from an older lung.

>>>the vertebrate lung has a different embryological and evolutionary origin
>than the fish gill, and started off as a blind-ended outpocketing of the
>anterior portion of the gut.  It has been such a structure ever since, with
>only one opening to the outside and with air coming in and out of the same
>opening.  Subsequent modifications have been primarily to the pouch and not
>the connecting tube; the original paired single chambers have been
>subdivided and subdivided again, primarily as an attempt to increase the
>diffusional surface area. <
>I'll open myself up to correction again and ask why an (arguably) close to
>optimal gas exchange sytem (gills), which evolved in a fluid medium that was
>approximately 50% oxygen (H2O),

The oxygen in the H2O molecule is not available for gas exchange.  It is
the dissolved O2 that gills extract.  (H20 dissociates into H+ and OH-, the
latter being a very stable free radical).

The problem is simply that fish style gills have problems when not kept
immersed in water.  This is reinforced by the evolution of land breathing
in snails and spiders.  In spiders the formerly exposed gills of their
aquatic ancestors became enclosed in pockets within the abdomen, replacing
the free, counter-current system of marine chelicerates with a closed,
two-way flow, system called book lungs.  (In marine arthropods, the limbs
on the abdominal segments take the form of gills).

Similarly, terrestrial snails modified their gills into closed-sack
structures, also called lungs (ergo, Pulmonata as the name for terrestrial
snails).  Though in snails this is less of an change, as even the aquatic
ones have their gills enclosed inside the shell and mantle.

May the peace of God be with you.         sarima@ix.netcom.com